Abstract The goal of this research program is to develop a novel effective adoptive immunotherapy for the treatment of poorly differentiated thyroid cancers such as anaplastic thyroid cancer (ATC) or poorly differentiated thyroid cancer (PDTC); both of them have very poor prognoses. This goal stems from the following pieces of information: i) no effective therapy is available for anaplastic thyroid cancer, ii) we have found for the first time that ATC and PDTC thyroid cancer cells express the tumor antigen (TA) chondroitin sulphate proteoglycan 4 (CSPG4), which is an attractive target of antibody-based immunotherapy; and iii) we have available in our laboratory the reagents required to develop adaptive immunotherapy of undifferentiated thyroid cancer with T cells genetically engineered to express a TA-specific chimeric antigen receptor (CAR). We have selected CAR T cells as effector cells, since this strategy allows rapid generation of polyclonal T cells with TA-specificity and potent cytotoxic activity. It is noteworthy that CAR T cells have already been used for treatment of thyroid cancer in an experimental setting. The target antigen used is ICAM1. Therefore, the positive results obtained cannot be translated to a clinical setting given the broad expression of ICAM 1 in a number of normal tissues. We have selected CSPG4 as a target, since this antigen is highly expressed on malignant cells including anaplastic thyroid cancer cells, but has a restricted distribution in normal tissues. According to the information in the literature and according to our own extensive data, CSPG4 is only detectable on activated pericytes in the tumor microenvironment. As a result, immune targeting of CSPG4 is expected to selectively not only inhibit tumor cells but will also inhibit neo-angiogenesis in the tumor microenvironment, contributing to the elimination of thyroid cancer cells, even those which do not express CSPG4, without the side effects associated with the systemic administration of anti-angiogenic drugs. In preliminary experiments we have found that CSPG4 CAR T cells can recognize thyroid cancer cells, but they are not very effective in eradicating them. We believe that these results reflect the escape mechanisms utilized by thyroid cancer cells. Given the limited time and funding provided by the RO3 grant mechanism, in this proposal we plan to get some information which will support our hypothesis that CSPG4 CAR T cell based immunotherapy is a valid therapeutic approach for the treatment of ATC or poorly differentiated thyroid cancer (PDTC). Specifically, we will determine the frequency and clinical significance of CSPG4 expression in thyroid cancer. PDTC or ATC cell lines and human tissues and provide data on the clinical significance. In addition, we plan to test whether the antitumor activity of CSPG4 CAR T cells can be enhanced by strategies that counteract the escape mechanisms utilized by thyroid cancer cells.